It has been proposed to produce a thin coating film on a metal pipe by conducting a preheated pipe through a coating chamber in which the pipe is exposed to a turbulent air stream charged with a metered quantity of a coating powder.
In such a process it is desired that the synthetic-resin coating should adhere very firmly to the metal surface and form a smooth and homogeneous film having as uniform a thickness as possible over the entire coated surface.
In order to meet these requirements it has already been proposed to bring the pipe that is to be coated into contact with a pulverulent coating material which will fuse on the hot surface of the pipe, by depositing the coating material after it has been fluidized in an ascending turbulent gas stream. Upon raising of the gas pressure as is conventional when depositing liquids, the fusible coating material is carried from a storage container over the pipe located above the container, and a coating is thus formed.
Another previously proposed process provides elongate articles, particularly pipes, with coatings by the fluidized-solids sintering technique, the pipe which is to be coated being preheated and horizontally passed through a fluidized solids sintering container provided in its sides with entry and exit openings for the travelling pipe, a pressure above the fluidized bed being maintained below that of the ambient atmosphere.
It has also been proposed to coat the external surface of a pipe with an infusible resin by a process in which a pulverulent organic resin which is temporarily softened by heat is fluidized and thus deposited on the preheated pipe. The resin employed in this process is a mixture of epoxy resin and hardener.
These various processes and the apparatus employed permit pipes to be provided with homogeneous coatings which adhere firmly to the pipe surface.
Nevertheless, considerable difficulties are still experienced when very long pipes of very small internal cross-section are to be provided with synthetic-resin coatings of uniform thickness.
Pipes of small diameter can be easily and quickly heated but the applied heat is just as quickly dissipated, and it is no simple matter to provide such pipes during the process of coating with heat at a uniform rate to maintain the desired temperature in order to produce a coat that is of the same thickness everywhere.
It is also difficult to coat long pipes which stand upright with a turbulently suspended synthetic-resin powder because, although a coating of satisfactory thickness may be produced at the bottom end of the pipe, pressure loss of the fluidizing gas results along parts of the pipe higher up and particularly at the top, resulting in a coating of inadequate thickness. These difficulties can be somewhat alleviated if preheated pipes are conducted horizontally through a fluidizing container. However, it is still not easy to provide the fluidizing container with seals suitable for the passage of pipes of different diameters. Moreover, it is also not easy to maintain a constant pressure below that of the ambient atmosphere in the space above the fluidizing bed.
However, the principal shortcoming of these and other known fusion-coating techniques is that they do not permit the application of thin synthetic-resin (of a thickness below say 200 .mu.m) to thin-walled pipes. In order to permit this to be successfully done, the coating must be formed by a synthetic-resin powder having a grain size under 80 .mu.m., but such grain sizes are unsuitable for fluidization, for instance in fluidized-solids sintering, because the powder will not then settle. All the above mentioned fluidization techniques require powders having grain sizes between 80 and 250 .mu.m..
In principle electrostatic powder spraying of, for example, epoxy resins followed by hardening (K. Brown, Ind.-Lackier-Betr., 38 (1970)) permits thin coatings to be applied to pipes, but these and similar methods can be performed only at low speeds of travel of the pipes that are being coated. Even if a large and costly number of spray guns are used and simultaneously operated uniform coating films cannot be successfully obtained, primarily due to the electrostatic attraction between the powder grains.